[Paul McDermott]"The on camera audio and the Mp3 Recorder audio sound fine individually but seem to be playing back at fractionally different speeds."

Each recording device has an internal crystal that it uses as a clock to determine timing. No two clocks will ever be perfectly in sync. The accuracy of this clock is directly proportional to the price of the recording unit. Cheap units use cheap inaccurate crystals. Judging from the cost of your recording device, I'd say this is your problem. I use a Marantz PMD660 which holds sync with my Sony HVR-Z1U camera for about 15-20 minutes but eventually all independent recorders will drift unless you sync them with external GenLock.

I realize that you can't put a large recorder in the grooms pocket but at least now you know why it won't keep sync with your camera. You'll need to either stretch the audio to match or keep slicing it and repositioning it to stay in sync (or both).

John's answer is entirely correct, but I'll throw in another thing you can try.

Make sure your sample rates match. Your camera is probably recording at 48KHz, but your recorder might be recording at 44.1KHz. Matching them is just math, but the more math conversions you introduce into the problem, the greater the chance some mismatch could be introduced.

Since most video standards use 48k audio, try on your next shoot to adjust the groom's portable recorder to a 48KHz sample rate. I'll be curious to see if it makes a difference for you.

[Paul McDermott]"The problem is when I sync my camera audio and the MP3 recorder audio, the sync works fine for about half a minute but then an echo appears.

As the recordings go on they become more and more out of sync.

The on camera audio and the Mp3 Recorder audio sound fine individually but seem to be playing back at fractionally different speeds."

This was a problem even for expensive recording devices, early on.

When you record sampled audio, the expectation is that you're going to record at 48kHz or 44.1kHz samples, or whatever. The reality is that there's a little crystal in that device, usually a quartz oscillator (even worse when it's not) that has an accuracy rating. Typical crystals found in PCs and MP3 players, perhaps 100ppm (parts per million). At the edges of that accuracy, you're adding or dropping one 24p frame every 6.9 minutes, versus a perfect clock (I wrote about this awhile back here. And cheap consumer gear... sometimes they use ceramic resonators, which might have 1/5th the accuracy, 500ppm or worse.

Even companies you might not expect have had problems in this. Zoom (aka Samson) are pretty well regarded in the DSLR video world as the go-to gadget for "recorder on the flash shoe" devices (my H4n also has a 4-track "portastudio" mode, which makes it useful for recording song demos on vacation). But the first generation of these didn't lock up for more than about 10 minutes... pretty terrible. But also not something every DSLR shooter noticed, since the early cameras only shot for ten minutes or so.

But if, like me, you used audio in other ways, you noticed this. In shooting events, I like lots of cameras, so I always have one audio recorder on for the whole time, and use that as the sync track for everything else. That used to be MiniDisc, I switched over to flash recorders after that first generation got it wrong. Of course, most of these manufacturers were thinking "audio". They expected folks to use these instead of tape decks. They didn't expect all the love from video people. So today's hold sync pretty well. But when it's broken, it's broken.

Until you fix it, anyway. There are two techniques. Once you figure out the direction of the inaccuracy, you can time-stretch the recorder audio to match the camcorder audio. Most DAWs and I think Vegas comes with "time stretch without pitch changes" type plug-ins. The other is to slice up your recorder audio at short intervals, always at zero crossings of the audio. If you use a tool like PluralEyes to sync, that's not all that much trouble.

That CAN also be done by hand. I did that on a few music projects, when I had multiple takes of a performance but without a common time base (metronome, drummer playing to a reference track, etc), and really needed both takes. Success at nearly hopeless tasks like this are what make me seem so laid back and zen these days ...